1. Field of the Invention
The present invention relates to the field of hydrodynamic bearing assemblies of the type, which provides support and rotation for a high-speed spindle element. More specifically, the present invention relates to an improved apparatus for reducing variations in spindle stiffness and power as a function of temperature.
2. Background of the Invention
Disc drive memory systems have been used in computers for many years for storage of digital information. Information is recorded on concentric memory tracks of a magnetic disc medium, the actual information being stored in the form of magnetic transitions within the medium. The discs themselves are rotatably mounted on a spindle, the information being accessed by means of transducers located on a pivoting arm, which moves radially over the surface of the disc. The read/write heads or transducers must be accurately aligned with the storage tracks on the disc to ensure proper reading and writing of information; thus the discs must be rotationally stable.
During operation, the discs are rotated at very high speeds within an enclosed housing by means of an electric motor, which is generally located inside the hub or below the discs. One type of motor in common use is known as an in-hub or in-spindle motor. Such in-spindle motors typically have a spindle mounted by means of two ball bearing systems to a motor shaft disposed in the center of the hub. One of the bearings is typically located near the top of the spindle, and the other near the bottom. These bearings allow for rotational movement between the shaft and hub, while maintaining accurate alignment of the spindle to the shaft. The bearings themselves are normally lubricated by grease or oil.
The conventional bearing system described above, however, is prone to several shortcomings. First is the problem of vibration generated by the balls rolling on the raceways. Ball bearings used in hard disc drive spindles run under conditions that generally guarantee physical contact between raceway and ball, in spite of the lubrication layer provided by the bearing oil or grease. Hence, bearing balls running on the generally smooth but microscopically uneven and rough raceways transmit this surface structure as well as their imperfection in sphericity in the form of vibration to the rotating disc. This vibration results in misalignment between the data tracks and the read/write transducer. This source of vibration limits the data track density and the overall performance of the disc drive system.
Another problem is related to the application of hard disc drives in portable computer equipment, resulting in severely increased requirements for shock resistance. Shocks create relative acceleration between the discs and the drive casing, which in turn show up as a force across the bearing system. Since the contact surfaces in ball bearings are very small, the resulting contact pressures may exceed the yield strength of the bearing material, and create long-term deformation and damage to the raceway and the balls of the ball bearing.
Moreover, mechanical bearings are not easily scaleable to smaller dimensions. This is a significant drawback since the tendency in the disc drive industry has been to continually shrink the physical dimensions of the disc drive unit.
As an alternative to conventional ball bearing spindle systems, researchers have concentrated much of their efforts on developing a hydrodynamic bearing. In these types of systems, lubricating fluidxe2x80x94either gas or liquidxe2x80x94functions as the actual bearing surface between a stationary base or housing in the rotating spindle or rotating hub of the motor. For example, liquid lubricants comprising oil, more complex ferromagnetic fluids or even air have been utilized in hydrodynamic bearing systems. The reason for the desirability of the use of air is the importance of avoiding the outgassing of contaminants into the sealed area of the head/disc housing. However, air does not provide the lubricating qualities of oil. The relatively higher viscosity of oil allows for larger bearing gaps and therefore looser tolerance standards to achieve similar dynamic performance.
A common type of fluid dynamic bearing comprises a shaft extending through the sleeve or hub with one or more radially extending plates supported from the shaft. A fluid dynamic bearing is provided between the shaft and the bore through the hub, with the fluid, which occupies the gap between the inner surface of the bore and the outer surface of the shaft providing the stiffness for the shaft. Without this stiffness, the shaft is prone to tilting or wobbling over the life of the motor. As a result, any hub or disc supported for rotation by the shaft is prone to wobbling or tilting. Any such tilting or instability in the hub or disc would make reading or writing of data on the disc surface very difficult, and diminish the life of the motor and the disc drive in which it is used.
However, the very fact that a conventional fluid dynamic bearing design relies on the use of a fluid in a very narrow gap between a shaft and surrounding bore for establishing and maintaining radial stiffness creates a problem due to the substantial range of temperatures over which the motor must operate. In known journal bearing designs for the shaft, the temperature of the fluid when the system is at rest may be about 5xc2x0 C.-25xc2x0 C. depending on the temperature of the surrounding environment; in operation, the fluid temperature can be 70xc2x0 C. or more. Clearly, the viscosity of the fluid will change with the fluid becoming less dense and providing substantially less stiffness for the shaft. Thus, unless elaborate systems are incorporated into the design, it is very difficult to maintain the desired level of radial stiffness for the shaft over the entire range of operating temperatures of the disc drive.
Efforts have been made to modify the fluid used in the fluid dynamic bearing gap to minimize the changes in viscosity with changes in temperature; but such fluids can add to the cost of the bearing and motor, and have not fully achieved the goal of temperature compensation over a wide range of temperatures.
Disc drive speeds continue to increase due to obvious benefits in performance. Higher speeds usually means higher power requirements, but every effort is made to avoid increases in power due to power supply limitations and excessive heat that is generated. As a result of these constraints, high speed spindles need to be exceptionally efficient at all temperatures. Fluid dynamic motors tend to have high power requirements at low ambient temperature due to change in viscosity of oil. The resultant low temp power demand is often excessive so much attention has been devoted to reducing this low temperature power requirement.
It is therefore an objective of the present invention to provide a hydrodynamic bearing design, which is simple and reliable in design, while incorporating means for compensating for temperature variations while maintaining the radial stiffness of the system.
It is a further objective of the invention to provide a fluid dynamic bearing design which minimizes the changes in power consumption during long-term operation.
Another objective of the invention is to provide a design wherein the high speed spindle is very efficient at all operating temperatures.
The present invention is also intended to achieve a minimization in the variation in steady state run current between different operating temperatures.
These and other objectives of the invention are achieved by providing a insulator between at least a portion of the sleeve or bearing seat which surrounds the fluid dynamic bearing, and the hub and surrounding environment in which the bearing, typically a spindle motor operates.
More particularly, according to a preferred design of the present invention, a hub and shaft are provided which are mounted for relative rotation by providing two conical bearings spaced apart along the shaft, and a bearing seat facing each conical bearing. Fluid is maintained in the gap between each cone and the facing bearing seat, supporting the cone and seat for relative rotation. The outer surface of the bearing seat is insulated from the remainder of the motor by a thermal insulator which extends at least part way along the outer surface of the seats. This insulator is effective at keeping the bearings warm even in a relatively low temperature environment in which the motor may be used. The insulator may comprise a cylindrical ceramic or similar low thermal conductivity material extending at least part way along the axial distance outside of the bearing cones. In an alternative embodiment, an air space may be defined in the outer surface of the bearing seat, extending at least part way between the bearing cones.
The embodiments described above comprise means for insulating the bearings from the remainder of the surrounding motor, keeping the bearings warm even in a low temperature environment and causing the bearings to heat up more quickly by a reduction in the thermal mass ie, the area around the bearings which may heat up.
Because the reduction in thermal mass is recognized herein as having beneficial effects, it is also possible to use ceramic for the male cone which is mounted on the shaft; the female cone or bearing seat which surrounds the cone on the shaft; and potentially the shaft itself to reduce the thermal mass further.
Other features and advantages of this invention may be apparent to a person of skill in the art who studies the following description of a preferred embodiment given with reference to the attached drawings.